The invention relates to an apparatus and a method for controlling a shunt wound d.c. motor.
Shunt wound d.c. motors are the type of motors generally preferred for driving centrifuges and ultra centrifuges. Centrifuges and ultra centrifuges are used generally for separating sample substances in a liquid medium. Under certain conditions, particularly in the case of simple separations of two substances, the acceleration and braking rates are of secondary importance. However, where zonal, swing-out or vertical rotors are used with density gradients, very smooth acceleration and deceleration are of great importance, particularly at speeds between 0 and about 1200 rpm. In the case of acceleration, the density gradient is sufficiently stable after this starting phase as a result of the centrifugal force produced, thus the rotor can be brought to the final speed with full acceleration. Smooth starting acceleration can be dispensed with if self-forming gradient material is used. However, smooth deceleration is absolutely essential in order to avoid any undesirable mixing effects which would occur on maximum braking.
Known centrifuge drive motors usually have a fixed acceleration and the braking function can be switched in with a fixed braking effect. Consequently, the starting and braking rates are dependent upon the rotor mass. Where uncalibrated regulators are used, reproducibility is poor. None of the known centrifuges enables the braking slope to be regulated reproducibly independently of the rotor mass. Nor are there any known method of regulation of the braking rate below and above free deceleration. The use of lightweight and high-strength materials in the manufacture of rotors therefore precludes suitability for this type of application of density gradient centrifugation.
The object of the present invention is to provide a motor control system without the above disadvantages.
According to the apparatus and method of the present invention, the armature current is regulated with full field excitation during acceleration. During deceleration, the field current is regulated with full armature current in accordance with a signal having a defined adjustable slope.